The present invention relates to a belt drive unit for transmitting drive between the drive shaft of an internal combusion engine and associated accessories. Drive units of the type covered by the present invention essentially comprise a pin, designed to fit on to one end of the said shaft, and a pair of pulleys having different diameters and turning round the said pin. The first smaller-diameter pulley is fitted on to the said pin with a free-turning wheel inbetween, whereas the second is fitted on to the same pin with an electromagnetic coupling inbetween.
Units of the type described are used for powering a drive belt connected to a number of pulleys, each designed to control an accessory on the engine, such as the alternator, water pump, conditioner, power steering and similar. The said drive unit provides for powering the said belt at two different speeds, high or low, depending on whether the drive shaft is running over or below a preset speed respectively.
Drive units of the type described usually comprise a coupling connected, e.g. by means of a splined connection, to the said pin and provided with a pair of seats for two rolling bearings. Such units are also provided with a second coupling, coaxial with the first and also having a pair of seats for the said bearings, which are mounted between the said two couplings so as to sustain rotation of the second in relation to the first. Between the said couplings, provision is also made for a free-turning wheel. The first pulley on the drive unit, usually made of sheet metal, is secured in any convenient manner, e.g. welded, to the second coupling.
The second pulley on the drive unit, also usually made of sheet metal, is supported in relation to the said pin by appropriate components forming part of the said electromagnetic coupling.
Drive units of the type described involve a number of drawbacks. Firstly, the numerous parts and components involved, e.g. the said two couplings and rolling bearings, result in a complex, cumbersome construction. Secondly, a number of the said parts involve strict machining tolerances and considerable care during assembly.